1. Field of the Invention
The invention relates to a sensor and is primarily concerned with a sensor for sensing temperature and/or humidity of ambient air.
2. Discussion of Prior Art
One of the problems with sensors used to sense temperature and/or humidity is that of maintaining accuracy in the presence of strong sunlight. It is known to install sensing devices in ventilated housings such as a Stevenson Screen which are coloured white to minimise the effect of sunlight. However, white makes the sensor highly visible which is not always desirable.
When measuring temperature and/or humidity, a flow of air over the sensing device should ideally be provided. In the Stevenson Screen, air flows through the ventilated housing but relies on a slight wind to achieve that. With no significant air movement on a sunny day, the temperature of the interior of the housing can rise giving an inaccurate record of ambient temperature. Whilst it has been proposed to utilise a powered fan to provide a flow of air over a sensing device to reduce that problem, the use of a fan is undesirable in dust and sand environments and can become noisy and unreliable.
An object of the present invention is to provide an improved sensor which will help to overcome the above problems.
According to the invention there is provided a sensor comprising a tube having a first portion and a second portion, a shielding member having a cavity therein, and at least one sensing device for sensing a characteristic of ambient air, wherein the first portion of the tube is positioned within the cavity of the shielding member and the second portion of the tube is arranged to protrude from the cavity, there being a clearance between the outer surface of the first portion of the tube and the surface of the shielding member surrounding the cavity such that heat applied to the first portion of the tube causes air to flow through the tube in a direction from the first portion towards the second portion thereby drawing fresh air into the cavity through the clearance between the tube and the shielding member and wherein the sensing device is located within the cavity of the shielding member.
The tube produces a chimney effect as the second portion heats up as a result of, say, solar radiation, causing the air to flow even in the absence of a light wind. Moreover, as the sensing device is arranged within the shielding member it is shielded from direct sunlight.
The sensing device may be arranged on the first portion of the tube and is preferably arranged on the inside of the first portion of the tube. In that way, the sensing device is particularly well shielded from direct sunlight.
Preferably, the cavity has a closed lower end.
The lower portion of the tube has a first end arranged adjacent the closed end of the cavity.
Although the downward flow of air between the tube and shielding member will help to insulate the sensing device from solar radiation where the sensing device is arranged on the tube, it is desirable to produce the shielding member from an insulative material such as plastics so that the shielding member does not readily heat up and radiate heat across the clearance space to the sensing device.
Preferably, the tube is made from a relatively heat conductive material, most preferably the material is at least partly metallic.
Optionally, the tube may be fluted to provide maximum heat transfer.
The sensing device preferably senses meteorological air characteristics such as temperature or humidity.
If desired, a plurality of sensing devices can be provided. For example, two sensors may be provided for sensing temperature and humidity respectively.
In a preferred embodiment, the cavity and the tube are arranged to be substantially coaxial and preferably define an annular space between them.